While the screenshot does detail what, and how much is being traded, it still does not show that Arcus (the skype user) says he received the 250m. Only you say its been accepted. This ambiguity, combined with the fact that you don't have any screenshots ingame, means that there isn't sufficient evidence for this element. Thus, there's honestly nothing I can do I'm sorry.
If you were scammed and don't have the proof, I'm really sorry. I understand that because you've traded with him in the past, you felt comfortable, in future trades, to not demand proof that you would have otherwise obtained. In future however, to avoid situations like this, just request a PM and take all the screenshots (use gyazo or something).
Unless you have further proof (reply with it), I'll close this thread.